1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, more particularly to a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer of which transparency or color tone is reversibly changeable depending upon the temperature thereof, thereby recording information therein and erasing recorded information therefrom repeatedly as desired. The reversible thermosensitive recording may be used in information recording devices in any form, for instance, in the form of a card, a disk, a label, or a disk cartridge. The present invention also relates to a method of producing the above reversible thermosensitive recording medium. The present invention also relates to a method of image formation and erasure, using the reversible thermosensitive recording medium. The present invention furthermore relates to an apparatus for performing the above method of image formation and erasure, using the reversible thermosensitive recording medium.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images therein and erasing the same therefrom when such images become unnecessary. For example, as disclosed in Japanese Laid-Open Patent Application 55-154198, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as a vinyl chloride--vinyl acetate copolymer.
However, such a conventional reversible thermosensitive recording material has a shortcoming that a temperature range in which the recording material exhibits light transmission or transparency characteristics or is in a transparent state (hereinafter referred to as the transparentizing temperature width) is as narrow as 2 to 4.degree. C., so that it is difficult to control the temperature for performing such image formation while utilizing the properties of reversibly becoming light shielding or opaque or milky white.
With this shortcoming of the above reversible thermosensitive recording material taken into consideration, the inventors of the present invention previously facilitated image erasure (making images transparent) by using a mixture of a higher fatty acid and an aliphatic dicarboxylic acid to broaden the transparentizing temperature width to about 20.degree. C. as described in Japanese Laid-Open Patent Applications 2-1363 and 3-2089. This method, however, has a shortcoming that the erasure cannot be sufficiently facilitated when the ambient temperature largely changes or when the heat application time for the erasure is short.
In order to improve such erasability, it is proposed to broaden the transparentizing temperature width by using a mixture of (a) a higher ketone or a fatty acid ester having a lower melting point than those of higher fatty acids, and (b) an aliphatic dicarboxylic acid or a saturated aliphatic bisamide as described in Japanese Laid-Open Patent Application 4-366682, 5-294062 and 6-255247. This method is capable of broadening the transparentizing temperature width and accordingly capable of improving the erasability. However, due to the use of the higher ketone or fatty acid ester having a lower melting point than those of higher fatty acids, the transparentizing temperature width is situated in a low temperature range, so that this method has a shortcoming that the formed opaque or milky white images formed are erased when the ambient temperature is high.
In order to improve the erasability of the image without lowering the heat resistance thereof, it has been proposed to shift the transparentizing temperature width to a high temperature side by using a mixture of (a) a low-molecular-weight compound having a low melting point and (b) an alicyclic dicarboxylic acid having a melting point of about 200.degree. C. which is significantly higher than the melting points of aliphatic dicarboxylic acids (as described in Japanese Laid-Open Patent Applications 5-139053, 6-48024 and 6-48025, or by using a mixture of (a) a low-molecular-weight compound having a low melting point and (b') a low-molecular compound having a steroid skeleton having a melting point near to 200.degree. C. (as described in Japanese Laid-Open Patent Applications 8-20167 and 8-282131). These recording media are capable of improving the erasability while maintaining the heat resistance of the image, but has the shortcomings that the temperature difference between a transparentizing upper-limit temperature and an opaqueness initiation lower-limit temperature is so large that a significantly large amount of energy is required for the formation of milky white images, and that the durability of the media is lowered while in repeated use, with the surface of the recording media scratched, and the opaqueness of the image lowered in the course of the repetition of image printing and erasure.
When a large amount of energy is required for the image formation, a thermal head's pulse application time is required to be lengthened since there is a limit to a voltage that can be applied to the thermal head from a power source, or the recording speed be lowered. Furthermore, when the amount of energy applied to the thermal head is increased, the life of the thermal head is shortened. Thus, when the amount of energy required for the image formation is increased, the applied energy has adverse effects on an apparatus using the reversible thermosensitive recording medium. In this case, it is considered that the high opaqueness initiation temperature is caused by the use of a low-molecular weight compound having an excessively high melting point.